Second-generation bioethanol from lignocellulosic biomass: An integration-oriented review of pretreatment, bioconversion, techno-economic assessment, and industrial perspectives

Abstract

The growing demand for sustainable energy and the environmental impacts of fossil fuel use have accelerated the development of renewable alternatives, particularly biofuels. Among them, second-generation (2G) ethanol produced from lignocellulosic biomass has emerged as a promising solution to overcome the limitations of first-generation (1G) ethanol, including competition with food resources and land-use constraints. However, efficient conversion of lignocellulosic biomass remains challenging due to its structural recalcitrance and process inefficiencies. This review critically examines recent advances in 2G ethanol production, focusing on pretreatment, enzymatic hydrolysis, and fermentation. Special attention is given to biomass structural complexity, its influence on conversion efficiency, and emerging strategies to improve sugar release and microbial fermentation. Recent studies indicate that optimized pretreatment and hydrolysis can achieve fermentable sugar recoveries of approximately 0.70–0.90 g sugars/g biomass, while the most advanced pretreatment technologies have reached Technology Readiness Levels (TRL) of 8–9, highlighting their industrial potential. The review also discusses biorefinery integration and co-product generation as strategies to improve economic feasibility and resource efficiency. Despite significant progress, important challenges remain, including inhibitor formation, high enzyme costs, and scale-up limitations. Technology selection should therefore consider feedstock characteristics, process integration, and techno-economic performance rather than individual process efficiencies alone. Unlike previous descriptive reviews, this work provides an integration-oriented framework that compares technological maturity, process integration, techno-economic performance, and industrial implementation pathways. By identifying current bottlenecks and future research priorities, it provides decision-support insights for developing more efficient, sustainable, and commercially viable lignocellulosic ethanol production systems.